This invention relates in general to ground penetrating radar systems, and more particularly to the concurrent use of multiple transducers for a ground penetrating radar system (GPR).
There is a growing demand for GPR systems that have the ability to acquire data with more than one transducer. The ability to run more than one transducer at a time is extremely complex given the nature of the problem. Systematic control of accurate timing in a distributed multitransducer network of GPR systems requires unique timing and logic elements.
In current practice, systems most often have one transmitter and one receiver transducer. Generally GPR systems obtain data along a measurement traverse line with the transmitter and receiver in a fixed geometrical configuration with respect to one another (prior art, FIG. 1); the GPR system as a whole is moved over the ground or medium to be explored (Annan, A. P., Davis, J. L., Ground Penetrating Radarxe2x80x94Coming of Age at Last, 1997; Proceedings of the Fourth Decennial International Conference on Mineral Exploration (Exploration""97), Toronto Canada, Sep. 14 to Sep. 18, 1997).
References to the utilization of more than one transmitter or receiver are limited. Prior attempts have been made as described in U.S. Pat. No. 5,248,975 issued to Schutz, A. E., entitled xe2x80x9cGround Probing Radar with Multiple Antenna Capabilityxe2x80x9d.
There are four major problems that have to be overcome.
The first problem is that the acquisition of ground penetrating radar traces in single transient waveform capture process, in digital form (or even analog form) is virtually impossible. Current commercially available analog to digital (A/D) converters are simply not fast enough nor do they have sufficient dynamic range to record the signals required for many of the GPR applications.
As a result, GPR systems resort to some sort of repetitive signal in order to capture the desired data. The most common approach is to use equivalent time sampling. Other approaches are to use a step frequency continuous sinusoidal wave technique that acquires data in the frequency domain by detecting the in-phase and quadrature response of the transfer function at a number of frequencies; the time domain signal is created by fourier transform.
A third approach is to use a fast A/D converter with few bits (i.e. limited dynamic range) and then stack the resultant signal for many repetitions in order that the resolution can be brought up. A fourth approach is to transmit some stream of random signals and use a correlation technique to extract the impulse response.
With all these approaches, considerable time is needed at each observation point to acquire data of a satisfactory nature. Combining such complex, individual signal capture processes with multiple spatially distributed transducers and simultaneously maintaining timing synchronization to very tight tolerances is a complicated task. The complexity arises is part arises in part because the transit time to transfer control signals between spatially separated transducers is both finite and are comparable or bigger than the measurement time lags.
The second major problem in trying to operate more than 1 unit is that multiple 2 transmitting sources operating at the same time can interfere with one another. If one wishes to operate two units, which are collecting independent information but operating at the same time then it is important that the signals from the transmitters do not get emitted at exactly the same time so that the two data sets can be acquired with high fidelity. In other words, a multiplexing process is required. In some instances it is desirable to have the transmitters operating simultaneously, but in this case one wants to make sure that the timing of the transmitters is synchronized in order to enhance the measurement process.
The third problem is that the transducers (or antennas) which create, emit and capture the electromagnetic signals which are transmitted into the ground are highly dependent on their immediate surroundings. When multiple transducers are placed in close proximity to one another, the transducers can interact in an almost unpredictable fashion and generate spurious signals.
The final problem is with the spatial distribution of the transducers. Since the signals that are being measured are radio waves that travel at the speed of light, all of the times involved in the measurement process are very short. Since the subsurface spatial dimensions may be similar to the separation distances between GPR components, the travel times on the inter connecting cabling or internal signal paths of the systems can become as large or larger than the travel times of the signals through the media being probed. As a result, it is important that any timing system be able to recognize these time differences and provide a means to measure and/or adjust times to eliminate the time delays associated with spatial distribution of the transducers.
FIGS. 2-5, show the most commonly envisaged multi-unit systems. FIG. 2 shows the use of multi transducer systems where the objective is to obtain data records from a variety of separations between the transducers. Many applications could benefit if data from a multiplicity of separations could be acquired simultaneously. Fisher, E., McMechan, G. A., and Annan, A. P., Acquisition and Processing of Wide-Aperture Ground Penetrating Radar Data; 1992; Geophysics, Vol. 57, p. 495-504, and Greaves, R. J. and Toksoz, M. N, Applications of Multi-Offset Ground Penetrating Radar; Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, 1994; (SAGEEP""94), p. 775-793 discuss the use of variable offset measurements and the enhancement of the data that can be achieved by coherent spatial stacking in the spatial dimension.
The acquisition of multiple separation data measurements made at each station along the transect line, is called multi-fold offset surveying. Multi-offset data available at every measurement point allows for the extraction of a velocity cross-section, an attenuation cross-section and an enhancement of data by determining an optimum spatial stacking velocity structure.
The second type of multi-channel system is depicted in FIG. 3. In this case the objective is to cover a larger area more quickly. Many GPR applications require acquisition of data on a series of parallel lines in order that a large area can be covered to obtain a three dimensional volume view of the ground.
One way of improving such surveys is to have a number of measurement systems mounted side-by-side and have these transported over the ground simultaneously. In FIG. 3a, a one channel system is shown sequentially measuring up and down 4 lines to acquire the same data that 4 transducers traversing once simultaneously over the four lines would achieve as shown in FIG. 3b. It is useful to note in this application that the individual units can more or less operate independently. They do not require synchronous sampling times but it is desirable that the transmitter be set up to operate at different staggered times to eliminate any potential of interference between the units caused by simultaneous operation of the individual units.
FIG. 4 depicts still another type of application where multiple transducers or measurements are desirable. The bandwidth of GPR systems is limited by the intrinsic characteristics of antennas. For detailed study of the subsurface, a number of systems with different frequency bandwidths and corresponding different physical sizes may have to be traversed along the same line in order to achieve full coverage of the subsurface.
At present, this type of operation is achieved by surveying the line a number of times as depicted in FIG. 4, once with each transducer. The whole operation could be completed more quickly if all (three transducers in the example shown) transducers are moved simultaneously along the line at one time and the same data acquired. Coordination of spatial acquisition and signal acquisition timing on a moving platform is commonly required for speed and efficiency of data acquisition.
The most general use of multi-unit systems is depicted in FIG. 5 and consists of a full array of transmitters and receivers spread over an area. The operation of transmitters either independently or synchronously together in time, as well as all of the receivers operating and acquiring data synchronized in time, provides a powerful means of subsurface imaging. The whole package shown could be transported along the line to provide multi-offset continuous data in a three dimensional fashion. Such data acquisition then lends itself to use of synthetic aperture processing or the equivalent multifold three-dimensional 3D seismic processing concepts that are commonly applied in the petroleum industry.
Such an application requires precise synchronization of the timing of all of the transmitters and receivers that are spatially distributed. If the platform is moving in space then synchronization of platform position and data acquisition time is an added factor to be managed.
Equivalent time sampling (ETS) is a means of using multiple repetitions of a transient signal to capture a transient waveform (Mulvey, John, Sampling Oscilloscope Circuits; 1970; Internal Publication of Tektronix, Inc., Beaverten, Oreg. and Phillips). Other modes of operations such as continuous wave, step frequency or instantaneous capture and stacking can use the timing control concepts outlined here. We will use ETS to demonstrate concepts of the patent.
As indicated previously, ETS receivers require successive repetitions of the signal waveform to be recorded in order that it can be acquired. Fisher (supra) provides information on ETS and some of the types of systems that have evolved.
Analog ETS systems were spawned in the 1960""s and 1970""s. FIG. 6 depicts a typical ETS. A timing circuit is required which will provide a very controlled time delay between signal creation and the time at which a measure of the signal waveform (sampled over a short time interval) is acquired. Historically two analog ramps, one slow and one fast, were used to drive a comparator that would provide a time delayed trigger output.
For the ETS shown in FIG. 6, the key feature is that the receive trigger is delayed in time progressively on every repetition of the transmit pulse. This time delay is dictated by a control clock delay, increases the delay from a minimum value to a maximum value over a fixed amount of time (i.e., N repetitions of the control clock). When the number of desired repetitions of the control clock which span the time window to be swept has been reached, the whole system is reset and the sequence starts over again. To work properly the control clock has to be very stable and regular.
Using a sample and hold or a sampling head circuit, the transient signal is captured over a short interval in time and is output from the sampling device as a continuous analog voltage. Provided the control clock is stable and the delay time varies linearly, the analog voltage is a replica of the transient waveform input but which is slowed down in time. Time stretching of 1,000:1 or even 1,000,000:1 is common.
The captured signal in the case shown in FIG. 6 requires N repetitions of the master clock and the transmitted signal to acquire one replica of real signal. The real time transient waveform will be sampled over a real time interval Nxcex94t where xcex94t is the amount the receiver trigger is delayed on each successive cycle of the system. What characterizes such a system is the repetition rate. This is the clock shown in the schematic diagram in FIG. 6. If the repetition period of the clock is P, then the real time signal interval Nxcex94t will be acquired in an elapsed time of NP. This is an equivalent time stretch factor that is determined by the ratio       P          Δ      ⁢              xe2x80x83            ⁢      t        .
When using analog oscilloscope displays or audio tape recorders for data acquisition, the analog signal is stretched to the audio frequency range from the radio frequency range. This enables data display recording and replay using lower-cost and lower speed electronics.
The basic analog ETS system as depicted can be used to support multiple transmitters or receivers. If the triggering signals can be sequenced by a computer, or some sort of preprogrammed logic array, then a number of channels of data can be acquired as shown in FIG. 7.
In this situation the receiver and transmitter triggers as shown in FIG. 6 are fed through a switching network which enables transmitter or receiver units to be switched or enabled or disabled. The output of the receivers are analog traces which can then be digitized or displayed on an oscilloscope or recorded on an analog tape (Mulvey, John, supra).
There are drawbacks in this approach. If there are M transmitter and receiver pairs to be switched, then the acquisition time increases to Mxc3x97NP. In other words, data acquisition rate is slowed down. If a single transmitter and a multiple set of receivers are to be used to acquire time synchronous data, then the full waveform recording sequence for the receivers must be required before switching to another transmitter and repeating the sequence. Such multiplexing reduces the rate at which the system can be moved spatially.
There is no simple way in which the timing associated with delays along the interconnect lines can be handled in any systematic fashion. This may be developed into the system by calibrated cables or may be handled in post acquisition but it is not readily accommodated by the analog ETS configuration shown.
Therefore a multi transducer ground penetrating radar system in a compact self-contained modular form is needed.
This invention is a modular control system to enable time and space synchronized GPR data acquisition from multiple transducers. One aspect of the present invention is to provide an improved multi transducer capability for a ground penetrating radar system where a virtually unlimited number of transducers can be accommodated without the drawback of increased data acquisition time.
Another aspect of the present invention, allows for a completely operational, self-calibrating multi transducer system. The present invention contains modular compact circuits for internal timing of signal emission, detection, digitalization and recording of data. In addition, with suitable control logic, measurement of and compensation for inter transducer communications delays can be automated.
Conveniently, the present invention allows for independent operation of individual transducers but simultaneously permits acquired data to be used in a common process or by several independent acquisition and display systems.
Another aspect of the present invention is the ability to use a master timing computer to coordinate groups of the multi transducer subsystems to acquire data in an interleaved fashion but with each subsystem operating in a totally self contained manner controlled by its own computer or clock. This mode of operation is optimal when there is signal coupling between the subsystems but where the data from each subsystem can be treated as independent data streams.
Conveniently the time bases of the present invention can be synchronized such that all the devices can detect and record signals from all other devices. Operation in this manner is beneficial for enhancement and extraction of information contained in the spatial placement of the transducers. The ability to process all signals coherently allows for the implementation of real time or post acquisition synthetic aperture and multifold signal processing such as used in the petroleum seismic.